湘南印支期塔山岩体地球化学特征及形成构造背景

Geochemistry and tectonic setting of Indosinian Tashan pluton,southern Hunan Province

塔山岩体位于华南造山带北缘，形成于晚三叠世。自早至晚由粗中-中粒斑状二云母二长花岗岩、中细粒-细粒斑状二云母二长花岗岩、细粒二云母二长花岗岩组成，具块状构造。岩石具有高硅（SiO2=71.57%-75.01%）、富铝（Al2O3=13.92%-15.51%）、中钾（K2O=3.51%-5.20%）、中碱（Na2O＋K2O=6.89%-8.62%）、高ASI（1.12-1.52）的特征，总体属铁质、高钾钙碱性系列强过铝质花岗岩类。微量元素中Ba、Nb、Sr、Ti表现为明显亏损，Rb、（Th＋U＋K＋Ta）、（La＋Ce）、Nd、（Zr＋Hf＋Sm）、（Y＋Yb＋Lu）等则相对富集。稀土总量较低（17.9-184.0μg/g），轻稀土富集（（La/Yb）N=2.52-17.89），具明显的负Eu异常（δEu=0.17-0.37）。岩体具有较高的ISr值（0.73271和0.72739）和较低的εNd （t）值（-11.05和-10.82），两阶段Nd模式年龄（t2DM）为1.87 Ga和1.89 Ga。C/MF-A/MF图解显示源岩主要为泥质岩石，部分为长英质岩石。强过铝花岗岩样品的Al2O3/TiO2比值大部分小于100。上述地球化学特征表明塔山岩体为S型花岗岩，源岩主要为中、上地壳酸性岩石。构造环境地球化学判别图解显示为后碰撞构造环境。基于岩石成因、构造环境及构造演化过程，推断继中三叠世后期印支运动陆内强挤压之后，晚三叠世进入挤压松弛的后碰撞环境，地壳减压重熔而形成了塔山岩体。此外，地壳重熔可能还受到软流圈地幔上隆及其热传递的影响。

The Late Triassic Tashan granitic pluton located in the northern South China orogenic belt is composed of early stage coarse-medium-grained to medium-grained porphyritic biotite-dimicaceous monzogranite, medi-um-fine-grained to fine-grained biotite-dimicaceous monzogranite and late stage fine-grained biotite-dimica-ceous monzogranite. All the rocks have massive structure. The rocks are rich in silicon （SiO2=71.57-75.01 wt%）, aluminium （Al2O3=13.92 -15.51 wt%）, potassium （K2O=3.51 -5.20 wt%）, alkali and （Na2O＋K2O=6.89 - 8.62 wt%）, high ASI value （1.12-1.52）, thus belong to ferric, high-K calc-alkaline seriese strong perluminous grani-toids. Ba, Nb, Sr, P and Ti are pronounced depleted, while Rb, （Th＋U＋K）, （La＋Ce）, Nd, （Zr＋Hf＋Sm） and （Y＋Yb＋Lu） are comparatively concentrated. It displays lower total REE content（ΣREE=17.9-184.0 ppm）, en-richment of LREE （（La/Yb）N=2.53-17.89）and negative Eu anomaly （δEu=0.17-0.37）. It also has slightly higher ISr （0.73271 and 0.72739） but lowerεNd（t） （-11.05 and-10.82） with Nd model ages（t2DM） of 1.87 Ga and 1.89 Ga. The C/MF-A/MF diagram indicates that the granites are mainly originated from argillaceous rocks, some as the felsic rocks. Al2O3/TiO2 values of most strong perluminous granite samples are less than 100. All above geo-chemical characteristics indicate that Tashan granite belongs to S-type granitiods and suggest that the magma should come mainly from acid rocks of the middle and upper crust. The discriminant figures of tectonic setting show that the granites were formed in post-collisional tectonic setting. According to the petrogenesis, discrimina-tion of structural environment and regional tectonic evolutional setting, the formation mechanism of Tashan pluton is inferred as follows：the thickening of crust in Indosinian Movement during Middle Triassic caused the rise of the crust temperature, and later weakening of stress in post-collisional tectonic setting during Late Triassic caused the melting of middle-upper crust, and the magma emplaced in relative extentional environment. In addition, up-welling and heat transference of asthenosphere mantle might have played an important role in the forming of Tashan granitic magma.